This invention relates generally to rotation sensors and particularly to ring laser gyroscope rotation sensors. This invention particularly relates to apparatus and methods for preventing degradation of laser intensity caused by ionic conduction between a ring laser gyroscope body and the gas plasma.
A ring laser gyroscope employs the Sagnac effect to measure rotation. Two counterpropagating light beams in a closed path will have transit times that differ in direct proportion to the rotation rate about an axis perpendicular to the plane of the path. In a ring laser gyroscope the closed path is defined by mirrors that direct the light beams around the path. The mirrors must be precisely aligned to direct the light beams around the closed path. The mirror surface must be free of impurities to provide a laser beam intensity that will result in a usable signal.
The closed path is typically in a cavity formed in a frame or body that is formed of a glass ceramic material. The preferred glass ceramic material has a near zero coefficient of thermal expansion over the operating temperature range of a ring laser gyroscope. The glass ceramic preferred for ring laser applications is formed of a lithium alumino-silicate (LAS) material. The cavity is evacuated and then filled with a mixture of helium and neon, which is the gain medium for the laser.
In a typical application, a mounting device is used to mechanically attach the ring laser gyroscope body to a chassis or system block, which is mounted to the device for which rotations are to be measured. In some applications the mounting device is mechanically dithered to reduce the effects of lock-in errors that occur when the rotation rate is near zero. The mounting device is typically formed of a metal and is electrically grounded.
The inventors have observed that ring laser gyro sensors typically have a significant decrease in laser beam intensity after about 1000 hours of operation at elevated temperatures. The processes that cause this phenomenon constitute a life-limiting mechanism for ring laser gyroscopes operating at normal operating temperatures.